The present invention relates to a projection lens, more particularly to refraction-type projection lenses for projecting a magnified image appearing on the cathode ray tube of a projection television system onto a separate large screen.
Projection television systems are usually provided with three monochromatic color projection tubes or CRTs for blue, green and red, each being equipped with a projection lens for projecting a monochromatic image on the CRT onto a screen so as to compose three monochromatic images, thereby displaying a magnified color image on the screen. Because of the narrow band widths of luminous spectra of the fluorescent substances contained in the fluorescent screens of the CRTs, it is less necessary for the CRTs, in particular for general or personal use, to be equipped with such projection lenses including a chromatic aberration correction lens.
Recent projection television systems, however, tend to be equipped with a high resolution projection lens. In addition to this trend, there is now attempted to project images appearing on three monochromatic color CRTs having a broader wave length range in each CRT's fluorescent layer for the purpose of obtaining more brightness on the screen and for the purpose of low manufacturing cost. For these purposes, the projection lens is unavoidably required to be able also to correct the chromatic aberrations.
On the other hand, a high resolution projection lens has the serious problem that the projection lens undergoes a change in focal length with a change of temperature, which impairs image quality. In view of the above, the conventional plastic projection lenses do not perform well as to chromatic aberration corrections and the change of focal length due to temperature changes.
Glass projection lenses, of which all the component lenses are made of glass materials, are more advantageous in regard to the above-mentioned problems. But it is difficult to produce aspherical optical surfaces on glass lenses, glass projection lenses always comprise at least seven to ten lens elements of spherical surface, resulting in undesirable manufacturing cost or selling price and heavy weight. Accordingly, there has been a strong demand in this field to develop a novel projection lens for use with high quality projection television systems which is able to solve the above problems and to satisfy the requirements of low price and light weight.
Responding to this demand, high resolution projection lenses have been developed by making use of conventional plastic lens technology in the production of projection lenses for use with high quality projection television systems. The key point is to use a plastic aspherical surface in the projection lens for the purpose of reducing aberrations of a monochromatic nature, with a decreased number of component lenses and light weight.
A plastic projection lens wherein all the component lenses are made of plastic materials, however, has the problem that it is hard to correct sufficiently the chromatic aberrations because of the limited varieties of available plastic materials for high quality lenses. In addition to this problem, there is the difficulty that plastic lenses are unable to serve as the principal elements of the projection lens because plastic materials have a large refraction index change with temperature and a high linear thermal expansion coefficient in comparison with glass materials as is shown, by example, in Table I, and there is no plastic lens whose shift of focal length with temperature falls within the permissible limits.
TABLE I ______________________________________ Refractive Index Change Linear Thermal Ex- with Temperature pansion Coefficient ______________________________________ Acrylic Resin -1.1 .times. 10.sup.-4 /.degree.C. 7.5 .times. 10.sup.-5 /.degree.C. Glass (BK7) 1.7 .times. 10.sup.-6 /.degree.C. 7.1 .times. 10.sup.-6 /.degree.C. ______________________________________
For these reasons, it is necessry to use some glass elements along with plastic elements in a projection lens for suitable correction of chromatic aberrations and for keeping the shift of focal length with temperature within the permissible limits.